As an optical information reproduction device, an optical memory system has been put to practical use in which an optical disk such as a CD (compact disc), a DVD, or a BD (Blu-Ray disc), an optical card, or the like is used as an information recording medium.
A reproduction principle of a conventional optical memory system is shown in FIG. 10 in a simplified form. FIG. 10 is an explanatory diagram showing a part of the configuration of a conventional optical information recording and reproduction device and a state in which information is reproduced from an information recording medium.
In the conventional optical information recording and reproduction device, reproduction light 107 from a light source (not shown in the figure) is condensed by an objective lens 110. The reproduction light 107 is irradiated, as irradiation light 105, on a recording mark 104, which is a recording region in which information is recorded, of a recording film 102 formed on a substrate 101 of an information recording medium 103. Reflected lights 106 and 108 from the recording mark 104 are detected, whereby the information is reproduced from the information recording medium 103. For example, if reflectance is set to be low if the recording mark 104 is in a recorded state and is set to be high if the recording mark 104 is an unrecorded state, the optical information recording and reproduction device can determine whether the recording mark 104 is in the recorded state or the unrecorded state and can reproduce optical information of the information recording medium 103 by detecting light amounts of the reflected lights 106 and 108.
However, in the optical memory system in the past, there is a problem in that the size (recording mark length) of a recording mark that can be reproduced is limited by an optical limit size called diffraction limit determined by an NA (numerical aperture) of the objective lens and the wavelength of reproduction light and a further increase in density is difficult.
In recent years, in order to eliminate the diffraction limit, an optical memory is proposed in which near-field light, a spot diameter of which can be set smaller than the diffraction limit, is used. Patent Literature 1 describes an optical recording and reproduction device in which a near-field light probe made of a microstructure of metal is used.
In the optical recording and reproduction device described in Patent Literature 1, a recording layer of a phase-change recording medium is subjected to phase change from crystal to amorphous by near-field light generated by the near-field light probe, whereby a recording mark is formed and recording is performed. The near-field light is irradiated on the phase-change recording medium on which the recording mark is formed and reproduction is performed by detecting a change in the intensity of scattered light returned from the phase-change recording medium. Since the near-field light is localized light exponentially attenuated as it is further apart from a generation source (light that does not propagate), in general, the near-field light cannot be extracted. However, a small fraction of the near-field light can be extracted as scattered light by bringing an object such as the recording mark close to the near-field light. Specifically, when the near-field light is irradiated on the recording mark of the phase-change recording medium, a rate of scattering of the near-field light changes according to presence or absence of the recording mark. Therefore, in the optical recording and reproduction device described in Patent Literature 1, reproduction is performed by detecting a change in the intensity of the scattered light from the recording mark.
However, as a result of examination, the inventors have found a problem in the optical recording and reproduction device described in Patent Literature 1 in that, although high-density recording can be performed by forming a recording mark having size close to the size of the near-field light (e.g., about several ten nanometers), it is difficult to satisfactorily reproduce a recording mark having the size of the near-field light.
According to the study by the inventors, besides low conversion efficiency from reproduction light (propagated light) into near-field light (in general, only about 1%), when size d of the recording mark is smaller than the diffraction limit and reduced to size of a so-called Rayleigh scattering region (with respect to wavelength λ, d≦˜λ/10, for example, when λ=405 nm, d≦˜40 nm), even if the near-field light can be condensed to size close to the size d of the recording mark and irradiated on the recording mark, a light amount of returning scattered light suddenly decreases as the size d is smaller. For example, the inventors have estimated through optical calculation that, in a recording mark having size of about 20 nm, a light amount of returning scattered light is, for example, about 0.001% with respect to a light amount of the irradiated near-field light and, with respect to a light amount of reproduction light, the light amount of the returning scattered light is only about 1% of the abovementioned value, i.e., about 0.00001% at most.
As a result, there is a problem in that a change in the intensity of scattered light detected according to presence or absence of the recording mark is, for example, smaller than 0.00001% with respect to the reproduction light amount and reproduction of information is difficult because the change in the intensity of the scattered light is too small.